File folders, and, in particular, hanging file folders, have long been used in standard storage units such as file cabinets, desk drawers, and the like, and are a necessary storage tool in modern offices and businesses. The typical folder is made of folded cardboard with metal support bars attached to the upper edges. The metal support bars have hooks or notches at their ends to enable the folder to hang on a complementary standard parallel file frame commonly provided in office storage equipment, such as the aforementioned file cabinets and desk drawers.
Such folders often are unable to stand up to the wear and tear to which they are subjected. The cardboard is susceptible to creasing and ripping, particularly where it contacts the support bars and/or along the center fold where the weight of inserted papers is concentrated. In addition, during heavy usage the metal support bars may bend out of shape, so that the folder can no longer be hooked onto the suspension rails. Manufacture of hanging file folders is also less efficient than it could be, due to the number of manufacturing steps needed to produce the cardboard folder and support bars and to attach the components to each other. Furthermore, it is difficult to recycle such a file folder, because of the need to remove the metal support bars from the cardboard prior to disposal.
One way to make a more durable file folder is to manufacture it from thermoplastic material. Thermoplastics such as polypropylene resins have been used in a variety of office products such as pocket folders and non-hanging file folders, which have advantages over cardboard folders such as durability and shape-retention. Thermoplastic folders are also easy to recycle.
In the manufacture of a hanging thermoplastic file folder, a thin thermoplastic sheet may be formed and folded to define a folder, and support bars, also formed of thermoplastic, may be positioned along the upper edges of the folder to add strength and rigidity, and to enable the folder to hang from suspension rails. One concern in forming such a folder is the method by which the relatively thick support bars are attached to the comparatively thin, upper edges of the folder. The desired result is a smooth, flat file folder with straight, fairly rigid bars along the upper edges. However, known methods for connecting a thick bar of plastic to a thin sheet of similar or like material often achieve less-than-satisfactory results.
Attaching or welding a thin sheet of plastic to a thick bar or strip is a technique practiced in a variety of technological fields. One such field involves the formation of plastic bags with zipper closures. U.S. Pat. No. 5,152,613 to Herrington (hereinafter "the '613 patent") discloses a "plastic film zipper bag having straightened heat seals", wherein a thin film of plastic is extruded and plastic zipper "fins" or elements are extruded in a separate operation. The plastic film, which forms the bag portion, is thereafter attached at its upper edges to the plastic fins of the zipper track. The plastic fins are thicker in cross-section than the thin film edges. Thus, when these components are heated and melted together, the thin film edges heat and melt more quickly than the fins, which act as a heat sink due to their much greater thickness. The unequal heating and melting of these components tend to cause the fin and film to shrink and pucker after they have been welded together and begin to cool down. The result is puckering and bending along the weld line. The '613 patent addresses this phenomenon by disclosing a method of stretching the seal line.
While welding together separate components with unequal cross-sections can lead to inadequate welds, certain other processes avoid such weld problems by forming the thick and thin sections together, i.e., as one piece. For example, in a process known as profile extrusion, the profile, or cross-section, of the part to be extruded contains both thick and thin sections. Typically, however, a profile design that contains both thick and thin sections is to be avoided, because the thicker section cools more slowly than the thin part of the profile upon exiting the profile extrusion die. As the profile cools, it shrinks somewhat. The relatively quick cooling and shrinking of the thin section, coupled with the slower cooling and shrinking of the thick section, can result in a profile whose shape is warped or distorted.
Also known in the prior art is a hanging, disk-storage pocket sold by the Esselte Corporation from 1983 to 1985. The disk-storage pocket was for storing 5.25-inch and 3.5-inch computer disks, and the transparent, PVC pocket could be hung on a support frame. The pocket has a single support bar that is high-frequency welded to a back upper edge of the pocket, and the front upper edge of the pocket is cut in a wide V shape to facilitate access to the inside of the pocket. To manufacture the disk-storage pocket, the support bar is extruded as a relatively stiff profile extrusion which is cut into individual support bars, and notches are cut into the bars near their terminal ends to enable the completed pocket to hang on a support frame. In a separate process, flat sheets of PVC are cut into appropriate shapes for the front and back walls of the pocket body. To assemble the pocket, a first flat sheet that is to form the back wall is aligned so that its upper edge overlaps an edge portion of the support bar. A second flat sheet, pre-cut to the shape of the pocket's front wall, is positioned on top of the first flat sheet. The first and second flat sheets are welded to each other at their side and bottom edges in order to form the pocket body, and the top edge of the first flat sheet is welded to the support bar to complete the pocket assembly.
The manufacturing process for making the disk-storage pocket thus requires a number of discrete steps, all of which add to the expense of producing the item. Furthermore, the completed pocket has exposed seam edges where the overlapping support bar and back wall edges are welded together. These exposed seam edges are capable of snagging computer disks or other items that are inserted into or pulled out of the pocket, and they create stress concentrations in the adjacent portions of the pocket walls when the pocket is pulled, twisted or bent. Such stress concentrations can lead to premature tearing, which is unsightly and shortens the useful life of the pocket.
In manufacturing a thermoplastic hanging file folder, which is to have thick support bars attached to the top edges of a thin, folded sheet, none of the processes described thus far optimizes the connection between the thin sheet and the thick bar. As described above, extruding thin and thick components separately and then welding them together causes a weld line that tends to pucker or bend. Furthermore, extruding thick and thin components together in a single piece or profile tends to cause warping or distortion. Such an extrusion design is also particularly problematic in the manufacture of hanging file folders, because the extruded thermoplastic webbing used in forming the folder body passes between and around rollers at different stations in the machinery of the extrusion line. If the support bar were extruded together with the webbing, the thickness of the support bars would interfere with the rollers that guide and pull the sheet or web of thermoplastic through the machinery. Also described above is the process of welding together the edges of a relatively stiff bar and a comparatively bendable flat sheet of thermoplastic, with the result that exposed seams edges may cause snagging or tearing. In manufacturing processes that require the fabrication, cutting, alignment and welding of a number of separate pieces of thermoplastic material, the inefficiencies built into such processes increase the costs of the final product.
Thus, a need exists for a method of making a hanging file folder that is more durable than the typical paper folder known in the art; that can be made of thermoplastic, wherein the manufacturing process does not lead to puckering, warping, distortion, snagging or tearing at the connection between the support bars and the upper edges of the folder; and wherein the number of manufacturing steps is minimized so as to produce an inexpensive, strong, recyclable folder having support bars that are securely and smoothly attached to the folder edges.